Three genes, PDF1.2 (At5g44420), KIN1 (At5g159600), and C2H2 Zinc Finger AZF4 (At5g04340), that represent pathogen response gene, stress-induced gene, and transcriptional regulator, respectively, were employed in this study to investigate the interaction between pathogen and stress signalling pathways. Using macroarray technique and Reverse Transcriptase Quantitative PCR, I first selected JA/ethylene signalling pathways as the model pathways as these are known to co-regulate the expression of pathogen resistance and stress response genes. Second, I examined the gene expression profiles in wild type and Arabidopsis mutants plants affected in jasmonic acid (jar 1-1), ABA (aba2-1, abi1-1 and abi2-1), and ethylene (etr1-1, ein2, and ein3) signalling, following treatment of signalling chemicals, i.e. MJ, ABA, and ethylene, Fusarium oxysporum inoculation and abiotic stress e.g. salt and drought treatments. The results identified cross talk between pathogen and stress signalling pathways at molecular level. The cross talk resulted in coordinated regulation of the expression of the PDF1.2, KIN1, and AZF4 genes.
This study confirms the previous finding that ethylene and jasmonic acid signaling concomitantly regulate PDF1.2 expression. Mutation in JAR1-1, ETR1-1, EIN2, and EIN3-1 all block the induction of PDF1.2 expression by methyl jasmonate and ethylene treatments as well as Fusarium infection. This study also revealed that PDF1.2 expression is negatively regulated by ABA and drought. The negative regulation may be mediated by deactivation of a protein phosphatase 2 A (PP2A) which is involved in JA signaling or by degradation of ethylene signaling component EIN3. Inhibition of PDF1.2 expression by ABA was not blocked by mutation in either ethylene signaling genes or the ABA synthesis gene ABA2, suggesting that the inhibition of PDF1.2 expression by ABA is not regulated in the same way as the ABA inhibition of germination and root growth. In addition, mutation in ABA2 inhibited the induction of PDF1.2 expression by ethylene treatments. This result supports the previous finding that ABA2 may be integrated with the ethylene signalling pathway. Similarly, mutation in abscisic acid insensitive genes ABl1/ABl2 inhibited the induction of PDF1.2 expression by methyl jasmonate indicating that the ABl1/ABl2 may act as positive regulator of jasmonic acid signalling as is required for the induction of PDF1.2 expression. Taken together, our results suggest that increased susceptibility of plants to pathogens upon ABA treatment or abiotic stress challenge may be due to the repression of JA- and ethylene-dependent pathogen response gene expression.
Results of this study confirm the previous reports that the KIN1 expression is induced by ABA, cold, drought treatments. The induction of KIN1 expression by ABA was reduced in abi1-1 and abi2-1 mutants. This indicates that ABl1 and ABl2 act as a positive regulator of KIN1 expression. In contrast, the induction of KIN1 expression is negatively regulated by ethylene signalling as indicated by the release of inhibition in ethylene signalling mutants. Mutations in ethylene sensitivity genes, i.e. etrl-1, ein2, and ein3-1 enhanced the expression of the KIN 1 gene upon ABA treatment. This result is also supported by results from not only drought treatment but also MJ treatment, salt stress and glucose-containing PDB media. Thus, KIN1 expression is positively regulated by ABA but negatively regulated by ethylene signalling, which mimics the role of these compounds in the regulation of seed germination. Therefore, it is possible that induction of ethylene during pathogen infestation may render the infested plants more vulnerable to osmotic stress.
AZF4 gene expression is induced ABA, MJ, and ethylene treatments. Induction of the AZF4 expression by those inducers was blocked by mutation in those signalling pathways. This study also demonstrated that induction of AZF4 by MJ requires botb JA and ethylene signalling pathways. This induction resembles that of pathogen signalling exemplified by the PDF1.2 gene in that strong induction requires the concomitant activity of JA and ethylene signalling. Meanwhile, ABA regulates the expression of AZF4 gene in a more complicated and possibly dose-dependent manner. At high ABA concentration, ABA interacts positively with ethylene signalling to control expression of AZF4 gene induction that resembles that of ABA inhibition of seedling root growth. In contrast to the gene expression response to exogenous application of MJ, ABA and ethylene, the response to salt and drought treatment indicates that ethylene signalling has a negative rather than positive effect on AZF4 expression. This mode of regulation resembles the interplay between ABA and ethylene in controlling seed germination. Regardless of specific mechanisms, these results show that AZF4 gene expression is controlled by three different modes of regulation.